The properties of some materials can be rather dramatically changed by the presence of relatively small quantities some elements, which may be present as impurities or dopants. For example, the electrical characteristics of a silicon-based semiconductor can be materially altered by the addition of oxygen or dopants such as arsenic or boron. Maintaining both the surface concentration and a concentration gradient of these elements in the semiconductor material within acceptable ranges is important in maintaining consistent product quality.
A variety of analytical techniques have been developed to characterize sample compositions on very small scales. For example, Energy Dispersive X-Ray Spectrometry (EDS) and Wavelength Dispersive X-Ray Spectrometry (WDS) have been used to analyze the surface of a sample in a non-destructive manner. Unfortunately, EDS, WDS and a number of other techniques are unable to reliable quantify material composition as a function of depth from the outer surface.
One analytical technique that does provide a depth profile of a sample's composition employs the use of an atom probe, which is essentially a time-of-flight mass spectrometer. In an atom probe, a surface of a sample is oriented toward, but spaced from, a detector plate. A high intensity electric field is established adjacent the sample surface. This electric field causes field evaporation of the atoms at the surface of the sample and these ionized atoms are drawn toward the detector. The electric field strength can be selectively pulsed for brief periods of time above the field strength necessary to evaporate atoms from the sample surface. This will establish a narrow range of time during which the atoms are released, allowing the time of flight for each atom to be determined for use in identifying the atom. In some atom probes, a local electrode is positioned proximate the sample to better concentrate the electrical field. Such so-called local-electrode atom probes (LEAPs) are discussed, for example, in U.S. Pat. No. 5,440,124, the entirety of which is incorporated herein by reference.
To achieve the required electric field density, the sample desirably has a very short tip that can be positioned immediately proximate the local electrode. The sharp point of this so-call “microtip” typically has a radius of curvature at its apex of less than 300 nm. These microtips may take the form of a sharpened tip at the end of an elongated acicular sample or a projection from a broader sample surface. In the latter sample type, an array of the projections may be arranged across at least a portion of the sample's surface, with each of the projections defining a separately evaporable microtip.
One technique for forming acicular samples involves growing the acicular projections on a substrate, e.g., by selected deposition on an etched wafer. Although this can be useful when attempting to analyze the material which is being deposited, it is of little value in analyzing the composition of an existing structure. Atom probe samples may also be mechanically grooved to form an array of acicular projections. In essence, this involves scribing the surface of the sample to a predetermined depth, e.g., using a diamond scribe or saw. Controlling the scribe or saw with sufficient precision to yield acicular projections of the requisite dimensions can prove quite difficult.
Ion beam mask etching has also been used to produce an array of microtips distributed essentially randomly across a sample surface. In this process, a number of particles of a material with a low sputtering rate, e.g., diamond or alumina particles, are scattered across the sample surface. The sample surface may then be eroded using an ion beam, e.g., by ion sputtering or reactive ion etching. The particles on the surface will shield a portion of the sample, leaving a cone beneath the particle. Continuing the ion beam exposure until the particles are also removed will yield an essentially random array of conical microtips, with each of the microtips being associated with the location of a particle on the original sample surface.
One shortcoming of ion beam mask etching is the difficulty of forming a microtip at a precise location. When examining the composition of an ion-doped area of a silicon wafer, for example, it may be difficult to precisely place a particle on the doped area to form a microtip at that location. Currently, the masking particle typically must be fabricated in place, e.g., using electron beam-fabricated carbon spikes. This relatively time-consuming multi-stage process requires specialized equipment and highly trained operators to yield reproducible results.